Glyoxal crosslinked cellulosic fibers having high bulk

ABSTRACT

Glyoxal crosslinked cellulosic fibers having high wet bulk, and methods for making the fibers.

FIELD OF THE INVENTION

The present invention relates generally to cellulosic fibers and, morespecifically, to glyoxal crosslinked cellulosic fibers having high bulk.

BACKGROUND OF THE INVENTION

Cellulosic fibers are a basic component of absorbent products such asdiapers. Although absorbent, cellulosic fibers tend to collapse onwetting, they retain absorbed liquid and consequently suffer fromdiminished liquid acquisition rate in absorbent products. The inabilityof wetted cellulosic fibers in absorbent products to further acquireliquid and to distribute liquid to sites remote from liquid insult canbe attributed in part to the loss of fiber bulk associated with liquidabsorption. An absorbent product's ability to acquire and distributeliquid will generally depend on the product's bulk and capillarystructure. The ability of a product to further acquire liquid onsubsequent insults will depend on the product's wet bulk. Absorbentproducts made from cellulosic fluff pulp, a form of cellulosic fibershaving an extremely high void volume, lose bulk on liquid acquisitionand the ability to further wick and acquire liquid, causing localsaturation.

Crosslinked cellulosic fibers generally have enhanced wet bulk comparedto uncrosslinked fibers. The enhanced bulk is a consequence of thestiffness, twist, and curl imparted to the fiber as a result ofcrosslinking. Accordingly, crosslinked fibers are advantageouslyincorporated into absorbent products to enhance their wet bulk andliquid acquisition rate and to also reduce rewet.

Because absorbent products ideally rapidly acquire liquid, effectivelydistribute liquid to sites remote from insult, continue to acquireliquid on subsequent insult, and have low rewet, there exists a need forcellulosic fibers having wet bulk sufficient to achieve these idealproperties. The present invention seeks to fulfill these needs andprovides further related advantages.

SUMMARY OF THE INVENTION

In one aspect, the present invention provides individualized cellulosicfibers having high wet bulk. The high wet bulk cellulosic fibers of theinvention are intrafiber crosslinked cellulosic fibers obtainable fromcellulosic fibers by treatment with glyoxal. The cellulosic fibers ofthe invention are crosslinked by treatment with only an aqueous glyoxalsolution (e.g., without a crosslinking catalyst). The fibers of thepresent invention have a wet bulk greater than about 20 cm³/g at 0.6kPa.

In another aspect of the invention, methods for the preparation ofcellulosic fibers having high wet bulk are provided. In the methods, afibrous web of cellulosic fibers is treated with only an aqueous glyoxalsolution (e.g., without a crosslinking catalyst), wet fiberized, andthen dried and cured to provide individualized cellulosic fibers havinghigh wet bulk.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention provides cellulosic fibers having high bulk andmethods for their preparation. The fibers of the invention are glyoxalcrosslinked cellulosic fibers. The glyoxal crosslinked cellulosic fibersare intrafiber crosslinked fibers. The glyoxal crosslinked cellulosicfibers are made by treatment with an amount of glyoxal effective toprovide crosslinked fibers having a wet bulk greater than about 20 cm³/gat 0.6 kPa.

As used herein, the term “bulk” refers to the volume in cubiccentimeters occupied by 1.0 gram of airlaid fluff pulp under a load of0.6 kPa. The term “wet bulk” refers to the volume in cubic centimetersoccupied by 1.0 gram (dry basis) of fluff pulp under load of 0.6 kPaafter the bulk has been wetted with water. Wet bulk underload ismeasured by the Fiber Absorption Quality (FAQ) Analyzer (WeyerhaeuserCo., Federal Way, Wash.) and reported in cm³/g at 0.6 kPa as describedbelow.

In one embodiment, the glyoxal crosslinked fibers have a wet bulkgreater than about 20 cm³/g at 0.6 kPa. In another embodiment, theglyoxal crosslinked fibers have a wet bulk greater than about 22 cm³/gat 0.6 kPa. In another embodiment, the glyoxal crosslinked fibers have awet bulk greater than about 25 cm³/g at 0.6 kPa. The bulk ofrepresentative glyoxal crosslinked cellulosic fibers of the invention asa function of crosslinking time and temperature is presented in Table 1.In addition to high bulk, the glyoxal crosslinked fibers of theinvention advantageous exhibit high brightness compared to other highbulk fibers. The bulk and brightness of representative glyoxalcrosslinked cellulosic fibers of the invention as a function ofcrosslinking time and temperature is presented in Table 2.

The high wet bulk cellulosic fibers of the invention are glyoxalcrosslinked cellulosic fibers. As used herein, the term “glyoxalcrosslinked cellulosic fibers” refers to cellulosic fibers obtainablefrom cellulose fibers by treatment with an aqueous glyoxal solutionwithout the use of a crosslinking catalyst. The glyoxal crosslinkedfibers of the invention are intrafiber crosslinked fibers.

The glyoxal crosslinked cellulosic fibers of the invention are made bytreating a mat or web of cellulosic fibers with an aqueous glyoxalsolution to provide glyoxal treated fibers, which are then separatedinto individual glyoxal treated fibers, and heated for a time and at atemperature to effect curing (i.e., to provide glyoxal crosslinkedcellulosic fibers). The glyoxal crosslinked fibers of the invention aremade by treatment with an aqueous glyoxal solution without the use of acrosslinking catalyst. Representative methods for making the glyoxalcrosslinked cellulosic fibers of the invention are described in Example1.

The preparation of cellulosic fibers crosslinked with glyoxal using acrosslinking catalyst is described in Example 2. The crosslinkingprocedure described in Example 2 is as described in Example 1 of U.S.Pat. No. 4,888,093, Individualized Crosslinked-Fibers and Process forMaking Said Fibers. The crosslinked cellulosic fibers prepared asdescribed in Example 2 had a bulk significantly lower than the bulk ofthe glyoxal crosslinked fibers of the invention. In contrast to theglyoxal crosslinked cellulosic fibers of the invention having a bulkgreater than about 20 cm³/g at 0.6 kPa, the highest bulk achieved forthe fibers crosslinked with glyoxal and crosslinking catalyst was about13 cm³/g at 0.6 kPa. The crosslinked cellulosic fibers prepared asdescribed in Example 2 had a brightness significantly lower than thebrightness of the glyoxal crosslinked fibers of the invention. Incontrast to the glyoxal crosslinked cellulosic fibers of the inventionhaving a brightness greater than about 80% ISO, the highest brightnessachieved for the fibers crosslinked with glyoxal and crosslinkingcatalyst was about 74% ISO.

In another aspect, the present invention provides cellulosic fiberscrosslinked by treatment with an aqueous glyoxal solution without theuse of a crosslinking catalyst. In the method for making glyoxalcrosslinked cellulosic fibers, cellulosic fibers to be crosslinked aretreated with an aqueous glyoxal solution. The fibers are treated with aneffective amount of glyoxal to achieve the wet bulk enhancementdescribed herein. Generally, the fibers are treated with from about 4 toabout 15 percent by weight glyoxal based on the total weight of thetreated fibers. In one embodiment, the fibers are treated with fromabout 6 to about 10 percent by weight based on the weight of fibers.

In general, the cellulose fibers of the present invention may beprepared by a system and apparatus as described in U.S. Pat. No.5,447,977 to Young, Sr. et al., which is incorporated herein byreference in its entirety. Briefly, the fibers are prepared by a systemand apparatus that includes a conveying device for transporting a mat orweb of cellulose fibers through a fiber treatment zone; an applicatorfor applying a treatment substance such as an aqueous glyoxal solutionfrom a source to the fibers at the fiber treatment zone; a fiberizer forseparating the individual cellulose fibers comprising the mat to form afiber output comprised of substantially unbroken and essentiallysingulated cellulose fibers; a dryer coupled to the fiberizer for flashevaporating residual moisture; and a controlled temperature zone foradditional heating of fibers and an oven for curing the crosslinkingagent, to form dried and cured individualized crosslinked fibers.

As used herein, the term “mat” refers to any nonwoven sheet structurecomprising cellulose fibers or other fibers that are not covalentlybound together. The fibers include fibers obtained from wood pulp orother sources including cotton rag, hemp, grasses, cane, husks,cornstalks, or other suitable sources of cellulose fibers that may belaid into a sheet. The mat of cellulose fibers is preferably in anextended sheet form, and may be one of a number of baled sheets ofdiscrete size or may be a continuous roll.

Each mat of cellulose fibers is transported by a conveying device, forexample, a conveyor belt or a series of driven rollers. The conveyingdevice carries the mats through the fiber treatment zone.

At the fiber treatment zone, an aqueous glyoxal solution is applied tothe cellulose fibers. The solution is preferably applied to one or bothsurfaces of the mat using any one of a variety of methods known in theart, including spraying, rolling, or dipping. Once the glyoxal solutionhas been applied to the mat, the solution may be uniformly distributedthrough the mat, for example, by passing the mat through a pair ofrollers.

After the fibers have been treated with the crosslinking agent, theimpregnated mat is fiberized by feeding the mat through a hammermill.The hammermill serves to disintegrate the mat into its componentindividual cellulose fibers, which are then air conveyed through adrying unit to remove the residual moisture. In a preferred embodiment,the fibrous mat is wet fiberized.

The pulp is then air conveyed through an additional heating zone tobring the temperature of the pulp to the cure temperature. The curetemperature for glyoxal is about 150° C. In one embodiment, the dryercomprises a first drying zone for receiving the fibers and for removingresidual moisture from the fibers via a flash-drying method, and asecond heating zone for curing the crosslinking agent. Alternatively, inanother embodiment, the treated fibers are blown through a flash-dryerto remove residual moisture, heated to a curing temperature, and thentransferred to an oven where the treated fibers are subsequently cured.Overall, the treated fibers are dried and then cured for a sufficienttime and at a sufficient temperature to effect crosslinking. Typically,the fibers are oven-dried and cured for about 3 to about 10 minutes at atemperature from about 135° C. to about 165° C. In one embodiment, thefibers are dried and cured at about 150° C. to about 157° C.

As noted above, the present invention relates to crosslinked cellulosefibers. Although available from other sources, cellulosic fibers usefulfor making glyoxal crosslinked cellulosic fibers of the invention arederived primarily from wood pulp. Suitable wood pulp fibers for use withthe invention can be obtained from well-known chemical processes such asthe kraft and sulfite processes, with or without subsequent bleaching.The pulp fibers may also be processed by thermomechanical,chemithermomechanical methods, or combinations thereof. The preferredpulp fiber is produced by chemical methods. Ground wood fibers, recycledor secondary wood pulp fibers, and bleached and unbleached wood pulpfibers can be used. The preferred starting material is prepared fromlong-fiber coniferous wood species, such as southern pine, Douglas fir,spruce, and hemlock. Details of the production of wood pulp fibers arewell-known to those skilled in the art. These fibers are commerciallyavailable from a number of companies, including Weyerhaeuser Company.For example, suitable cellulose fibers produced from southern pine thatare usable with the present invention are available from WeyerhaeuserCompany under the designations CF416, NF405, PL416, FR416, FR516, andNB416.

The wood pulp fibers useful in the present invention can also bepretreated prior to use with the present invention. This pretreatmentmay include physical treatment, such as subjecting the fibers to steamor chemical treatment. Although not to be construed as a limitation,examples of pretreating fibers include the application of fireretardants to the fibers, and surfactants or other liquids, such assolvents, which modify the surface chemistry of the fibers. Otherpretreatments include incorporation of antimicrobials, pigments, anddensification or softening agents. Fibers pretreated with otherchemicals, such as thermoplastic and thermosetting resins also may beused. Combinations of pretreatments also may be employed.

Method for Determining Fiber Wet Bulk, Absorption Time, Absorption Rate,and Capacity.

The wet bulk, absorption time, absorption rate, and capacity ofcellulosic fibers crosslinked with glyoxal of the invention wasdetermined by the Fiber Absorption Quality (FAQ) Analyzer (WeyerhaeuserCo., Federal Way, Wash.) using the following procedure.

In the procedure, a 4-gram sample of the pulp fibers is put through apinmill to open the pulp and then airlaid into a tube. The tube is thenplaced in the FAQ Analyzer. A plunger then descends on the fluff pad ata pressure of 0.6 kPa and the pad height bulk determined. The weight isincreased to achieve a pressure of 2.5 kPa and the bulk recalculated.The result, two bulk measurements on the dry fluff pulp at two differentpressures. While under the 2.5 kPa pressure, water is introduced intothe bottom of the tube (bottom of the pad). The time required for thewater to reach the plunger is measured. From this, the absorption timeand absorption rate are determined. The final bulk of the wet pad at 2.5kPa is also measured. The plunger is then withdrawn from the tube andthe wet pad allowed to expand for 60 seconds. The plunger is reappliedat 0.6 kPa and the bulk determined. The final bulk of the wet pad at 0.6kPa is considered the wet bulk (cm³/g) of the pulp product.

Method for Determining Fiber Brightness.

The brightness (% ISO) of cellulosic fibers crosslinked with glyoxal wasdetermined according to TAPPI T 525 om-02 on a Technibrite MicroTB-1Cinstrument (Technydine Corp.) As used herein, the term “brightness”refers to the reflectance of blue light corresponding to a centroidwavelength of 457 nm in terms of the perfect reflecting diffuser(perfect reflecting diffuser is the ideal reflecting surface thatneither absorbs nor transmits light, but reflects diffusely, with theradiance of the reflecting surface being the same for all reflectingangles, regardless of the angular distribution of the incident light).

The glyoxal crosslinked cellulosic fibers of the present invention canbe advantageously incorporated into an absorbent product to impart wetbulk to the product. Such products can further include other fibers suchas fluff pulp fibers, synthetic fibers, other crosslinked fibers, andabsorbent materials such as superabsorbent polymeric materials.Representative absorbent products that can include the glyoxalcrosslinked fibers of the invention include infant diapers, adultincontinence products, and feminine hygiene products. The glyoxalcrosslinked fibers can be included in liquid acquisition, distribution,or storage layers to provide products having superior liquid acquisitionrates, liquid distribution, and rewet properties. The glyoxalcrosslinked cellulosic fibers of the present invention can beadvantageously incorporated into tissue and towel products.

The glyoxal crosslinked fibers of the invention can be advantageouslyincorporated into paperboard products, including single and multi-plypaperboard products. Paperboard products that include the glyoxalcrosslinked fibers can be used in insulation applications, for example,insulated cups and containers. Paperboard products that include theglyoxal crosslinked fibers can also be used as packaging materials.

The glyoxal crosslinked cellulosic fibers of the invention areessentially odorless. This is in contrast to cellulosic fibers that havebeen modified with polycarboxylic acid crosslinking agents such ascitric acid crosslinked cellulosic fibers.

The bulk of representative glyoxal crosslinked fibers of the inventionas a function of glyoxal addition, cure temperature, and cure time issummarized in Table 1. TABLE 1 Bulk and Brightness for RepresentativeGlyoxal Crosslinked Fibers. Glyoxal Cure Temp. Cure time Bulk Brightness(%) (° C.) (° F.) (min.) (cm³/g) (% ISO) 6 140 284 5 21.2 87.4 6 140 2847 21.7 86.7 6 140 284 10 21.3 84.0 6 150 302 5 21.2 86.2 6 150 302 721.9 83.1 6 150 302 10 22.0 79.6 7 140 284 5 21.7 87.1 7 140 284 7 22.486.4 7 140 284 10 22.1 85.3 7 150 302 5 21.9 85.4 7 150 302 7 22.8 84.67 150 302 10 22.4 79.2 8 140 284 5 23.2 87.7 8 140 284 7 22.9 85.7 8 140284 10 23.2 84.2 8 150 302 5 22.8 85.7 8 150 302 7 23.6 83.5 8 150 30210 24.2 79.6 9 140 284 5 23.1 87.4 9 140 284 7 23.7 86.2 9 140 284 1024.6 85.6 9 150 302 5 23.0 85.2 9 150 302 7 24.8 82.7 9 150 302 10 24.980.5 10 140 284 5 23.8 87.9 10 140 284 7 24.3 85.9 10 140 284 10 25.985.0 10 150 302 5 24.2 86.0 10 150 302 7 25.3 83.7 10 150 302 10 26.379.8

The bulk and brightness of representative glyoxal crosslinked fibers ofthe invention as a function of glyoxal addition, cure temperature, andcure time are summarized in Table 2. TABLE 2 Bulk for RepresentativeGlyoxal Crosslinked Fibers. Glyoxal Temperature Time Bulk (%) (° C.)(min) (cm³/g) 3 135 5 17.9 3 150 5 18.3 3 150 7 18.4 3 150 5 18.6 3 1503 18.2 3 163 5 18.2 6 163 3 21.4 6 163 7 22.1 6 150 5 21.8 6 150 5 21.56 150 5 21.5 6 135 7 20.9 6 135 3 20.6 9 135 5 21.7 9 150 7 23.2 9 150 322.7 9 163 5 23.6

The present invention provides high wet bulk cellulosic fibers havingwet bulks greater than about 20 cm³/g at 0.6 kPa. The fibers of theinvention are intrafiber crosslinked cellulosic fibers obtainable fromcellulosic fibers by treatment with an aqueous glyoxal solution. Thefibers can be formed from cellulosic fibers by treatment with an amountof glyoxal without the use of a crosslinking catalyst effective toprovide the wet bulk enhancement described herein.

The following examples are for the purposes of illustrating, notlimiting, the present invention.

EXAMPLES Example 1 Representative Glyoxal Crosslinked Cellulosic Fibers

In this example, methods for forming representative high bulk fibers inaccordance with the present invention are described.

Method A.

A selected amount of glyoxal (CARTABOND GHF containing 40 precent byweight glyoxal in water) was applied to both sides of a twenty gram pulpsheet (NF405, dried wood pulp fibers available from Weyerhaeuser Co.)using a 5 mL disposable syringe and 23.1 gauge needle. The sample washeld in a resealable plastic bag for 16-18 hours at room temperature,then broken into pieces (e.g., about 2×2 cm), passed through alaboratory fiberizer, and collected as a loose pad. The pad was brokeninto small pieces (e.g., about 3×3 cm), placed into a screen basket andcured at a fixed temperature and time in a Despatch V Series oven.

Glyoxal crosslinked fibers prepared by this method had the bulkdescribed in Table 1.

Method B.

Pulp sheets in roll form (NF405, dried wood pulp fibers available fromWeyerhaeuser Co.) were treated with glyoxal (commercially availableunder the designation CARTABOND GHF from Clariant Corp.) according tothe following procedure. The pulp sheet was fed from the roll through aconstantly replenished bath of the crosslinking solution (i.e., anaqueous solution containing a glyoxal concentration determined by theweight add-on desired), then through a roll nip set to remove sufficientsolution so that the pulp sheet after treating was at about 40% moisturecontent. The concentration of the bath was adjusted to achieve thedesired level of chemical addition to the pulp sheet. After the rollnip, the wet sheet was fed through a hammer mill to fiberize the pulp.The individualized fibers were then blown through a flash dryer toaffect drying and then to a cyclone where the treated cellulose fluffwas separated from the air stream. The pulp was air conveyed through anadditional heating zone to bring the temperature of the pulp to the curetemperature and then transferred to an oven where the treated fiberswere subsequently cured.

Glyoxal crosslinked fibers prepared by this method had the bulk andbrightness described in Table 2.

Example 2 Comparative Example Bulk and Brightness for Cellulose FibersCrosslinked with Glyoxal Using Catalyst

In this example, the bulk and brightness of cellulose fibers crosslinkedwith glyoxal and catalyst is described. Wood pulp fibers (never-driedFR416 available from Weyerhaeuser Company) were treated with glyoxal andcrosslinking catalyst, zinc nitrate hexahydrate, 30 weight percent basedon the weight of glyoxal, according to the procedure described in U.S.Pat. No. 4,888,093, Individualized Crosslinked-Fibers and Process forMaking Said Fibers, Example 1. The only difference was that only one ofthe solutions was adjusted to 3.7 because all others were below thisvalue.

The fibers were treated with 3, 6, and 9 weight percent glyoxal andcatalyst, and cured (145° C., 45 min) to provide glyoxal crosslinkedfibers. The bulk (cm³/g) and brightness (% ISO) for the glyoxalcrosslinked fibers is summarized in Table 3. TABLE 3 Bulk and Brightnessfor Glyoxal/Catalyst Crosslinked Fibers Glyoxal Bulk Brightness (wt %)(cm³/g) (% ISO) 3 12.1 73.2 6 12.7 65.6  6* 12.7 67.4 9 13.2 60.6*The pH was adjusted to around 3.7 (as described in the ‘093 patent)with NaOH.The pH of the other three samples was 2.74, 2.50, and 2.27 as-is,respectively.

As shown in Table 3, bulk increases with increasing glyoxal amount, andbrightness decreases with increasing glyoxal amount.

While the preferred embodiment of the invention has been illustrated anddescribed, it will be appreciated that various changes can be madetherein without departing from the spirit and scope of the invention.

1. A method for preparing individualized, glyoxal crosslinked cellulosicfibers having a wet bulk greater than about 20 cm³/g at 0.6 kPa,comprising: (a) applying only an aqueous glyoxal solution to acellulosic fibrous web to provide a web of treated fibers; (b)separating the web of treated fibers into individual treated fibers; and(c) heating the individual treated fibers at a temperature and for atime sufficient to provide individualized, crosslinked cellulosic fibershaving a wet bulk greater than about 20 cm³/g at 0.6 kPa.
 2. The methodof claim 1, wherein the glyoxal is applied in an amount from about 4 toabout 15 percent by weight based on the total weight of fibers.
 3. Themethod of claim 1, wherein the glyoxal is applied in an amount fromabout 6 to about 10 percent by weight based on the weight of fibers. 4.The method of claim 1, wherein the temperature is from about 135° C. toabout 165° C.
 5. The method of claim 1, wherein the time is from about 3to about 10 minutes.
 6. The method of claim 1, wherein the crosslinkedfibers have a wet bulk is greater than about 22 cm³/g at 0.6 kPa.
 7. Themethod of claim 1, wherein the crosslinked fibers have a wet bulk isgreater than about 25 cm³/g at 0.6 kPa.
 8. The method of claim 1,wherein the crosslinked fibers have a brightness greater than about 80%ISO.
 9. A method for preparing individualized, glyoxal crosslinkedcellulosic fibers, consisting essentially of: (a) applying an aqueousglyoxal solution to a cellulosic fibrous web to provide a web of treatedfibers; (b) separating the web of treated fibers into individual treatedfibers; and (c) heating the individual treated fibers at a temperatureand for a time sufficient to provide individualized, crosslinkedcellulosic fibers.
 10. The method of claim 9, wherein the glyoxal isapplied in an amount from about 4 to about 15 percent by weight based onthe total weight of fibers.
 11. The method of claim 9, wherein thetemperature is from about 135° C. to about 165° C.
 12. The method ofclaim 9, wherein the time is from about 3 to about 10 minutes.
 13. Themethod of claim 9, wherein the crosslinked fibers have a wet bulk isgreater than about 20 cm³/g at 0.6 kPa.
 14. The method of claim 9,wherein the crosslinked fibers have a brightness greater than about 80%ISO.
 15. A method for preparing individualized, glyoxal crosslinkedcellulosic fibers, comprising: (a) applying only an aqueous glyoxalsolution to a cellulosic fibrous web to provide a web of treated fibers;(b) separating the web of treated fibers into individual treated fibers;and (c) heating the individual treated fibers at a temperature and for atime sufficient to provide individualized, crosslinked cellulosicfibers.
 16. The method of claim 15, wherein the glyoxal is applied in anamount from about 4 to about 15 percent by weight based on the totalweight of fibers.
 17. The method of claim 15, wherein the temperature isfrom about 135° C. to about 165° C.
 18. The method of claim 15, whereinthe time is from about 3 to about 10 minutes.
 19. The method of claim15, wherein the crosslinked fibers have a wet bulk is greater than about25 cm³/g at 0.6 kPa.
 20. The method of claim 15, wherein the crosslinkedfibers have a brightness greater than about 80% ISO.